A number of possible modulation/multiple access schemes are being considered for a physical layer of an ultra wide bandwidth (UWB) communications system by the IEEE 802.15.3 standards working group. One is multiband orthogonal frequency division multiplexing (OFDM).
As shown generally in FIG. 1, OFDM transmitters send data (OFDM symbols or ‘blocks’) 101 in parallel on K carrier frequencies (“tones”) 102. Note that for multiband OFDM, all of the carrier frequencies are within one band. For example, in one proposal to the IEEE 802.15 standards working group, the 128 tones are spaced approximately 4 MHz apart, and have a duration of approximately 300 ns. Each of the tones is quaternary phase shift keying (QPSK) modulated.
In practice, OFDM signals are not generated by multiple, separately modulated, local oscillators. Instead, an inverse fast Fourier transform (IFFT) is applied to the symbols to be transmitted as individual tones, e.g., see May et al., “Orthogonal Frequency Division Multiplexing,” Part IV, Molisch (ed.), Wideband Wireless Digital Communications, Prentice-Hall, pp. 309–385, 2001. The IFFT is typically implemented as a ‘butterfly’ structure, see, van Nee et al., “OFDM for Wireless Multimedia Communications,” Artech House, pp. 33–39, January 2000.
Such a transceiver is relatively complex. However, for some applications, the complexity of OFDM signaling may not be required at a receiver.
An alternative, less complex form of signaling is multiband frequency shift keying (FSK) within each band. Although FSK has usually a lower data rate, and perhaps, a lower quality, FSK signaling is much simpler to implement, reducing the cost of the receiver.
However, providing FSK signaling as an alternative signaling mode has drawbacks for a standardized implementation. Now, the transmitter is required to be able to transmit both high complexity OFDM signals and low-complexity FSK signals. That is, the transmitter must include two different types of modulators, i.e., both OFDM and FSK. This increases the cost of the transmitter.
It is desired to provide an OFDM transmitter that is able to transmit FSK signals without substantially increasing the cost.